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<a href="initialization.html" title="cpp/language/initialization"> Initialization</a><div class="t-navbar-menu"><div>
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<p>Binds a reference to an object
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="reference_initialization.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="reference_initialization.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="reference_initialization.html#Lifetime_of_a_temporary"><span class="tocnumber">3</span> <span class="toctext">Lifetime of a temporary</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="reference_initialization.html#Notes"><span class="tocnumber">4</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="reference_initialization.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="reference_initialization.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span>
</h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>=</b></code> <span class="t-spar">object</span> ;
<p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>=</b></code> <code><b>{ </b></code> <span class="t-spar">arg1, arg2, ...</span> <code><b>};</b></code>
</p>
<p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>(</b></code> <span class="t-spar">object</span> <code><b>) </b></code>;
</p>
<p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>{ </b></code> <span class="t-spar">object</span> <code><b>} </b></code>;
</p>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>=</b></code> <span class="t-spar">object</span> ;
<p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>=</b></code> <code><b>{ </b></code> <span class="t-spar">arg1, arg2, ...</span> <code><b>};</b></code>
</p>
<p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>(</b></code> <span class="t-spar">object</span> <code><b>) </b></code>;
</p>
<p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>{</b></code> <span class="t-spar">object</span> <code><b>} </b></code>;
</p>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <span class="t-spar">R</span> <span class="t-spar">fn</span> <code><b>(</b></code> <span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">arg</span> <code><b>);</b></code>
<p><span class="t-spar">or</span>
</p>
<p><span class="t-spar">R</span> <span class="t-spar">fn</span> <code><b>(</b></code> <span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">arg</span> <code><b>);</b></code>
</p>
<p><span class="t-spar">fn</span> <code><b>(</b></code> <span class="t-spar">object</span> <code><b>)</b></code>
</p>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td>
</tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">fn</span> <code><b>() { </b></code>
<p><span class="t-spar">or</span>
</p>
<p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">fn</span> <code><b>() { </b></code>
</p>
<p><code><b>return</b></code> <span class="t-spar">object</span> <code><b>;</b></code>
</p>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td>
</tr>




<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
<tr class="t-sdsc">
<td> <span class="t-spar">Class</span><code><b>::</b></code><span class="t-spar">Class(...)</span> <code><b>:</b></code> <span class="t-spar">refmember</span><code><b>(</b></code> <span class="t-spar">expr</span><code><b>)</b></code> {...
</td>
<td> (5)
</td>
<td class="t-sdsc-nopad">
</td>
</tr>

 }}

<tr>
<td colspan="10" class="t-sdsc-sep">
</td>
</tr>
</table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span>
</h3>
<p>A reference to <code>T</code> can be initialized with an object of type <code>T</code>, a function of type <code>T</code>, or an object implicitly convertible to <code>T</code>. Once initialized, a reference cannot be changed to refer to another object.
</p>
<p>References are initialized in the following situations:
</p>
<div class="t-li1">
<span class="t-li">1)</span> When a named <a href="reference.html#Lvalue_references" title="cpp/language/reference">lvalue reference</a> variable is declared with an initializer</div>
<div class="t-li1">
<span class="t-li">2)</span> When a named <a href="reference.html#Rvalue_references" title="cpp/language/reference">rvalue reference</a> variable is declared with an initializer</div>
<div class="t-li1">
<span class="t-li">3)</span> In a function call expression, when the function parameter has reference type</div>
<div class="t-li1">
<span class="t-li">4)</span> In the <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span></span></span> statement, when the function returns a reference type</div>
<div class="t-li1">
<span class="t-li">5)</span> When a <a href="data_members.html" title="cpp/language/data members">non-static data member</a> of reference type is initialized using a <a href="constructor.html" title="cpp/language/initializer list">member initializer</a>
</div>
<p>The effects of reference initialization are:
</p>
<ul><li> If the reference is an lvalue reference:
</li></ul>
<dl><dd>
<ul><li> If <span class="t-spar">object</span> is an lvalue expression, and its type is <code>T</code> or derived from <code>T</code>, and is equally or less cv-qualified, then the reference is bound to the object identified by the lvalue or to its base class subobject.
</li></ul>
</dd></dl>
<dl><dd>
<ul><li> If <span class="t-spar">object</span> is an lvalue expression, and its type is implicitly convertible to a type that is either <code>T</code> or derived from <code>T</code>, equally or less cv-qualified, then the non-explicit conversion functions of the source type and its base classes that return lvalue references are considered and the best one is selected by overload resolution. The reference is then bound to the object identified by the lvalue returned by the conversion function (or to its base class subobject)
</li></ul>
</dd></dl>
<ul><li> Otherwise, if the reference is either rvalue reference or lvalue reference to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span>:
</li></ul>
<dl><dd>
<ul><li> If <span class="t-spar">object</span> is an xvalue, a class prvalue, an array prvalue, or a function lvalue  type that is either <code>T</code> or derived from <code>T</code>, equally or less cv-qualified, then the reference is bound to the value of the initializer expression or to its base subobject.
</li></ul>
</dd></dl>
<dl><dd>
<ul><li> If <span class="t-spar">object</span> is a class type expression <span class="t-rev-inl t-since-cxx17"><span>that is not reference-related to <code>T</code> and</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> that can be implicitly converted to an xvalue, a class prvalue, or a function value of type that is either <code>T</code> or derived from <code>T</code>, equally or less cv-qualified, then the reference is bound to the result of the conversion or to its base subobject. <span class="t-rev-inl t-since-cxx17"><span>User-defined conversions are not considered</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>
</li></ul>
</dd></dl>
<dl><dd>
<ul><li> Otherwise, a temporary of type <code>T</code> is constructed and <a href="copy_initialization.html" title="cpp/language/copy initialization">copy-initialized</a> from <span class="t-spar">object</span>. The reference is then bound to this temporary. Copy-initialization rules apply (explicit constructors are not considered).
</li></ul>
</dd></dl>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Lifetime of a temporary">edit</a>]</span> <span class="mw-headline" id="Lifetime_of_a_temporary">Lifetime of a temporary</span>
</h3>
<p>Whenever a reference is bound to a temporary or to a base subobject of a temporary, the lifetime of the temporary is extended to match the lifetime of the reference, with the following exceptions:
</p>
<ul><li> a temporary bound to a return value of a function in a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span></span></span> statement is not extended: it is destroyed immediately at the end of the return expression. Such function always returns a dangling reference.
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx17">
<td>
<ul><li> a temporary bound to a reference member in a constructor <a href="constructor.html" title="cpp/language/initializer list">initializer list</a> persists only until the constructor exits, not as long as the object exists.
</li></ul>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td>
</tr> 
</table>
<ul>
<li> a temporary bound to a reference parameter in a function call exists until the end of the full expression containing that function call: if the function returns a reference, which outlives the full expression, it becomes a dangling reference.
</li>
<li> a temporary bound to a reference in the initializer used in a new-expression exists until the end of the full expression containing that new-expression, not as long as the initialized object. If the initialized object outlives the full expression, its reference member becomes a dangling reference.
</li>
</ul>
<p>In general, the lifetime of a temporary cannot be further extended by "passing it on": a second reference, initialized from the reference to which the temporary was bound, does not affect its lifetime.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span>
</h3>
<p>References appear without initializers only in function parameter declaration, in function return type declaration, in the declaration of a class member, and with the <code>extern</code> specifier.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span>
</h3>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;utility&gt;</span>
<span class="co2">#include &lt;sstream&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
 <span class="kw4">int</span> mi<span class="sy4">;</span>
 <span class="kw4">const</span> <a href="../utility/pair.html"><span class="kw904">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>,<span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy3">&amp;</span> mp<span class="sy4">;</span> <span class="co1">// reference member</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">void</span> foo<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
 
<span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span>
   <span class="kw4">int</span> n<span class="sy4">;</span>
   operator <span class="kw4">int</span><span class="sy3">&amp;</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
B bar<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="kw1">return</span> B<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">//int&amp; bad_r; // error: no initializer</span>
<span class="kw4">extern</span> <span class="kw4">int</span><span class="sy3">&amp;</span> ext_r<span class="sy4">;</span> <span class="co1">// OK</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
 <span class="co1">// lvalues</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r1 <span class="sy1">=</span> n<span class="sy4">;</span>  <span class="co1">// lvalue reference to the object n</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> cr<span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// reference can be more cv-qualified</span>
    <span class="kw4">volatile</span> <span class="kw4">int</span><span class="sy3">&amp;</span> cv<span class="br0">{</span>n<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// any initializer syntax can be used</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r2 <span class="sy1">=</span> r1<span class="sy4">;</span> <span class="co1">// another lvalue reference to the object n</span>
<span class="co1">//    int&amp; bad = cr; // error: less cv-qualified</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r3 <span class="sy1">=</span> <span class="kw1">const_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span>cr<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// const_cast is needed</span>
 
    <span class="kw4">void</span> <span class="br0">(</span><span class="sy3">&amp;</span>rf<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> foo<span class="sy4">;</span> <span class="co1">// lvalue reference to function</span>
    <span class="kw4">int</span> ar<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span>
    <span class="kw4">int</span> <span class="br0">(</span><span class="sy3">&amp;</span>ra<span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> ar<span class="sy4">;</span> <span class="co1">// lvalue reference to array</span>
 
    B b<span class="sy4">;</span>
    A<span class="sy3">&amp;</span> base_ref <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// reference to base subobject</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> converted_ref <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// reference to the result of a conversion</span>
 
<span class="co1">// rvalues</span>
<span class="co1">//  int&amp; bad = 1; // error: cannot bind lvalue ref to rvalue</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> cref <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// bound to rvalue</span>
    <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> rref <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// bound to rvalue</span>
 
    <span class="kw4">const</span> A<span class="sy3">&amp;</span> cref2 <span class="sy1">=</span> bar<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// reference to A subobject of B temporary</span>
    A<span class="sy3">&amp;&amp;</span> rref2 <span class="sy1">=</span> bar<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>      <span class="co1">// same</span>
 
    <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> xref <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// bind directly to n</span>
<span class="co1">//  int&amp;&amp; copy_ref = n; // error: can't bind to an lvalue</span>
    <span class="kw4">double</span><span class="sy3">&amp;&amp;</span> copy_ref <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// bind to an rvalue temporary with value 1.0</span>
 
<span class="co1">// restrictions on temporary lifetimes</span>
    <a href="../io/basic_ostream.html"><span class="kw1420">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> buf_ref <span class="sy1">=</span> <a href="../io/basic_ostringstream.html"><span class="kw1427">std::<span class="me2">ostringstream</span></span></a><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'a'</span><span class="sy4">;</span> <span class="co1">// the ostringstream temporary</span>
                      <span class="co1">// was bound to the left operand of operator&lt;&lt;, but its lifetime</span>
                      <span class="co1">// ended at the semicolon: buf_ref is now a dangling reference.</span>
 
    S a <span class="br0">{</span> <span class="nu0">1</span>, <span class="br0">{</span><span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// temporary pair {2,3} bound to the reference member</span>
                      <span class="co1">// a.mp and its lifetime is extended to match a</span>
                      <span class="co1">// (Note: does not compile in C++17)</span>
    S<span class="sy2">*</span> p <span class="sy1">=</span> new S<span class="br0">{</span> <span class="nu0">1</span>, <span class="br0">{</span><span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// temporary pair {2,3} bound to the reference</span>
                      <span class="co1">// member a.mp, but its lifetime ended at the semicolon</span>
                      <span class="co1">//  p-&gt;mp is a dangling reference</span>
    delete p<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<ul>
<li> <a href="default_initialization.html" title="cpp/language/default initialization">default initialization</a>
</li>
<li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a>
</li>
<li> <a href="value_initialization.html" title="cpp/language/value initialization">value initialization</a>
</li>
<li> <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>
</li>
<li> <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a>
</li>
</ul>

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